1. Field of the Invention
The present invention relates to a membrane-electrode assembly for a solid polymer electrolyte fuel cell and a solid polymer electrolyte fuel cell.
2. Description of the Related Art
A typical membrane-electrode assembly for a solid polymer electrolyte fuel cell includes a solid polymer electrolyte membrane having proton conductivity, a cathode electrode layer provided on one surface of the membrane, an anode electrode layer provided on another surface of the membrane, and gas diffusion layers stacked on these electrode layers. A solid polymer electrolyte fuel cell is formed by stacking separators, which also function as gas flow paths, on the gas diffusion layers of the membrane-electrode assembly having the above structure.
To improve the performance of solid polymer electrolyte fuel cells, it is necessary to reduce the thickness of a polymer electrolyte membrane as much as possible so as to improve proton conductivity. On the other hand, a problem of a decrease in the strength of the polymer electrolyte membrane occurs. In addition, sufficient strength and durability are desired for such a solid polymer electrolyte membrane so that breaking or the like does not occur even during long-term use, and thus various studies have been conducted.
FIG. 10 is a cross-sectional view showing the structure of a membrane-electrode assembly 100 for a solid polymer electrolyte fuel cell in the related art (refer to Japanese Unexamined Patent Application Publication No. 5-21077). In the membrane-electrode assembly 100, a frame-shaped protective film 102 composed of a fluorocarbon resin sheet is provided on at least one surface side of a solid polymer electrolyte membrane 101, and the solid polymer electrolyte membrane 101 and the frame-shaped protective film 102 are sandwiched between an anode electrode layer 103 and a cathode electrode layer 104. According to this membrane-electrode assembly 100, the solid polymer electrolyte membrane 101 and the frame-shaped protective film 102 overlap each other, and thereby the protective film 102 functions as a reinforcing member of the solid polymer electrolyte membrane 101. Consequently, the membrane-electrode assembly 100 can withstand an increase in the differential pressure and mechanical stress applied to this portion.
FIG. 11 is a cross-sectional view showing the structure of a membrane-electrode assembly 110 for a solid polymer electrolyte fuel cell in the related art (refer to Japanese Unexamined Patent Application Publication No. 2008-117775). In the membrane-electrode assembly 110, an absorption portion 118a is provided between an anode electrode layer 112 and a cathode electrode layer 114, the absorption portion 118a extending from edge portions 112a and 114a up to a certain distance. According to this membrane-electrode assembly 110, it is possible to suppress degradation of a polymer in a polymer electrolyte membrane layer 116 provided with a reinforcing layer 118, the degradation being due to hydroxy radicals generated by an electrochemical reaction.
However, in the membrane-electrode assembly 100 disclosed in Japanese Unexamined Patent Application Publication No. 5-21077, a dimensional change in the solid polymer electrolyte membrane 101 due to a change in the humidity may cause detachment at an interface between the solid polymer electrolyte membrane 101 and the protective film 102. In addition, the solid polymer electrolyte membrane 101 may become degraded in some cases and has a problem in terms of durability.
Also, in the membrane-electrode assembly 110 disclosed in Japanese Unexamined Patent Application Publication No. 2008-117775, although it is possible to suppress chemical degradation of the polymer electrolyte membrane layer 116 due to hydroxy radicals generated by an electrochemical reaction, a degradation due to a dimensional change in the polymer electrolyte membrane layer 116 caused by a change in the humidity may occur in a portion of the polymer electrolyte membrane layer 116, the portion being in contact with a sealing layer or adhesive layer used for stacking.